1. Field of the Invention
The present invention relates to a method of and a system for data communication in a communication network on an automobile, and more particularly to a method of and a system for data communication in a network of components of an audiovisual system on an automobile.
2. Description of the Prior Art
In recent years, automobile audio systems have been changing from systems which give a user only audio information such as music to systems which allow the user to enjoy both audio and visual information. Systems with both audio and visual capabilities are known as audiovisual systems.
Automobile audiovisual systems on automobiles are composed of many different audio and visual components. The audio components include a cassette tape deck, a radio tuner, a CD (compact disk) player, etc., while the visual component include a TV (television) tuner, a navigation system, etc.. Audio signals produced by some of these components are amplified by an amplifier and reproduced from loudspeakers in the automobile. Video signals produced by other components are displayed on a display unit in the automobile. Today, the audiovisual system components are controlled according to digital technology, i.e., by respective controllers in the form of microcomputers.
The components of an audiovisual system are required to be controlled systematically for systematic operation of the components. To meet this requirement, the controllers of the respective components are connected by a communication bus network, so that control data for the components will be transmitted through the communication bus.
In the conventional audiovisual system network, the controllers of the components are controlled by a polling process. According to the polling process, either one of the controllers is given a priority status and handled as a master unit, with the other controllers as slave units, and when the master unit is to collect data from a slave unit, the master unit always accesses the slave unit, i.e., initiates the transfer of the data from the slave unit.
When the master unit transmits communication data to a slave unit for access or the slave unit transmits data back to the master unit, it is necessary that the controllers be identified or specified. For this purpose, the controllers are allotted respective address indicating themselves.
Heretofore, the controllers are allotted respective unique addresses. Control data which are to be given to controllers comprise address data peculiar to the respective controllers and instruction data (e.g., start command: ON data) added to the address data. The control data which is generated is sent to the communication bus.
It is known that upon the transmission of communication data between the master and slave units, acknowledgement data (ACK) is sent back from a receiving unit to a transmitting unit in order to acknowledge that the data transmitted by the transmitting unit is properly received by the receiving unit. Depending on the degree of importance of the transmitted data, the transmitting unit may or may not confirm the acknowledgement data returned from the receiving unit. In audiovisual systems, there are a plurality of slave units, and communication data may contain a plurality of successive commands (e.g., start command, switching command, etc.) to be given to the slave units. According to synchronous transmission, the transmitting unit can relate a command to an acknowledgement code therefor by determining whether they are in timed relationship or not. According to asynchronous transmission, however, when a plurality of commands requiring respective acknowledgement codes are successively issued, the transmitting unit cannot ascertain which acknowledgement codes, returned from the receiving unit, correspond to which commands. Therefore, the transmitting unit is unable to determine whether the received acknowledgement codes should be confirmed or not.
One problem with the conventional communication process is that there are many complex communication data transmitted to and received from the communication bus. More specifically, commands contained in communication data which are transmitted from a master unit of an audiovisual system to slave units or components are fixed with respect to the slave units, and there are many such commands. When a new slave unit or component is to be added to the audiovisual system to expand the latter, commands also have to be added for use with the new component.
The communication data contains display data as well as commands to be issued from the master unit. Accordingly, the communication data, composed of these various commands and data, is complex and lengthy, and needs a complicated processing sequence in the transmitting unit.
Furthermore, when a new slave unit or component is connected to the communication bus, communication data to be sent to and received from the new slave unit may not be processed if its format is unknown.